I get this error when opening a larger project. I tried giving PL/SQL Developer (PSD) all available system resources, switched the preferences to single session as well, but still after a long time having the project loading I end up getting this error.

How can I open the project without getting such an error?
Why can

If you have many windows open for a large project, this may be consuming too many resources. Perhaps you can close some windows that you are not working on at the time? The corresponding items will of course remain part of the project, regardless whether they have a window open or not.

If you remove (or rename) the <project>.dsk file you should be able to open the project again. None of the project items will then have a window open.

The project shouldn't be consuming too many resources now when opening this project when it was once created with PL/SQL Developer like that successfully.
If PL/SQL Developer reaches windows resource limits it should handle those more properly and stop opening further windows as soon as it's risking running into such an error.
Just closing windows in this state is no longer possible! All I can do is close the whole project then.

Sure I can rename the dsk file and start with an empty window list that way. Then however I need to open windows from scratch again which is not the idea of a project. Also I won't know when I approach a limit which eventually will hit me with the same error again.
An application should just not work like this and leave its users to deal themselves with the problems it causes.

The project does not consume any resources by itself. Opening more than 100 windows will consume a lot of resources though, regardless whether they are opened in the context of a project, the file browser, the database or any other context that you use to organize your work.

PL/SQL Developer should limit the use of resources itself and stop opening further project windows when it cannot handle those anymore. Opening those anyway and then running into problems makes no sense!
It should be possible to open the same project also when there are less system resources available.
It should not be left to the user but instead be handled by the application to deal with the available resources wisely and still keep the application working acceptably.

We'll consider an option to limit the number of simultaneously opened windows. Perhaps we can implicitly close the oldest (least recently used) window that does not have any modifications.

I have added this to the list of enhancement requests.

Thank you.
Maybe another option would also be to not create DB-connections for windows exceeding resource limits.
Whenever such a window gets activated by the user then, it could automatically or manually reconnect, depending on preferences.

I guess a large part of resources is needed just for DB-connections, but those are not always necessary. Often you just need to look things up in other files and PL/SQL Developer is rather used like just an editor then.